Last December, a purported Islamic fighter called Amir Ghrozni posted a cryptic message on a password-protected website connected to al Qaeda.
"London, perhaps the punishment is near," the message said. "Await the glad tidings, as we will never calm down."
Similar threats to the 2012 Olympic Games, which begin in London on July 27, have surfaced since.
A February message on a different website urged readers to "be the initiator O Soldiers of Usama, as London has been beautified with its nicest decorations and the best of dresses."
In April, someone calling themselves "Kafir bel-Taghoot" reposted a 2003 religious tract by a militant Saudi Imam offering Islamic arguments legitimizing the use of weapons of mass destruction.
"Due to the nearing of the London Olympics...I gift you the holistic book that displeases the advocates of surrender and defeater of the advocates of reconciliation and tolerance," the message said.
British authorities remain concerned about the possible influence on home grown radicals of Anwar Awlaki, and American-born militant who was arguably the most influential English-language internet apostle of violent Jihad until he was killed in a CIA drone-strike last September at a hideout in Yemen.